In a typical wide-area wireless communications network (commonly referred to as cellular radio system), wireless terminals (also referred to as user equipment unit nodes, UEs, and/or mobile stations) communicate, through a radio access network (RAN), with one or more core networks, which provide connectivity to the public switched telephone network (PSTN) and/or to one or more data networks, such as the Internet. The RAN covers a geographical area that is divided into cell areas, with each cell area being served by a radio base station, also referred to, in various contexts, as a RAN node, a “NodeB”, or an enhanced NodeB (“eNodeB” or “eNB”). A cell area is a geographical area over which radio coverage is provided by the base station equipment at a (typically) fixed base station site. The base stations communicate, through radio communication channels, with UEs within range of the base stations.
In such a communications network, the maximum data rate that can be supported over a particular link between a base station and a wireless terminal may be limited by interference from other sources. As shown in FIG. 1A, for example, base station 19a may receive uplink communications from wireless terminals 11a and 15a. At a receiver of base station 19a, uplink communications from another wireless terminal 15a may interfere with reception of the uplink communications from wireless terminal 11a. A link of interest may be referred to as a “target link,” or “TL,” while a radio link that interferes with a link of interest may be referred to as an “interfering link,” or “IL.” A receiver that is attempting to receive (i.e., demodulate and decode) a TL is referred to as a TL receiver. Thus, in FIG. 1A, the uplink from wireless terminal 11a is referred to as “TL,” for “target link,” while the uplink communications from wireless terminal 15a is referred to as “IL,” for “interfering link.” It will be appreciated, of course, that the uplink from wireless terminal 11a may also interfere with those from wireless terminal 15a, so the designations “TL” and “IL” refer only to a particular demodulation scenario of interest, and may be applied to the same uplink in different scenarios.
A different interference scenario is illustrated in FIG. 1B. As shown in FIG. 1B, base station 19b-1 may transmit downlink communications to wireless terminal 11b, while base station 19b-2 is transmitting downlink communications to wireless terminal 15b. At a receiver of wireless terminal 11b (where the desired link is referred to as TL), downlink communications from base station 19b-2 to wireless terminal 15b (referred to as an IL) may interfere with reception of the downlink communications from base station 119b-1 to wireless terminal 11b. The dashed line in FIG. 1B indicates the IL as perceived/received at wireless terminal 11b. 
Still another interference scenario is illustrated in FIG. 1C. As shown in FIG. 1C, base station 19c may transmit downlink communications to wireless terminals 11c and 15c. At a receiver of wireless terminal 11c (where the desired link is referred to as TL), downlink communications from base station 19c to wireless terminal 15c (referred to as IL) may interfere with reception of the downlink communications from base station 119c to wireless terminal 11c. The dashed line in FIG. 1C indicates the IL as perceived/received at wireless terminal 11c. 
In any of the examples of FIGS. 1A, 1B, and/or 1C, a received signal at a TL receiver (at a wireless terminal or at a base station) may include a TL with information intended for the TL receiver and one or more interfering links. A ratio of received signal power of the TL to a received signal power of one or more ILs (as received at the receiver attempting to receive the TL) plus other noise and interference, may be referred to as a geometry factor. The geometry factor can be a significant factor determining an achievable data rate for the target link. Stated in other words, a greater geometry factor (i.e., a greater ratio of TL signal strength to IL signal strength at the TL receiver) may allow a greater data rate to be transmitted over the TL to the TL receiver than a lower geometry factor (i.e., a lower ratio of TL signal strength to IL signal strength at the TL receiver). By reducing an effective power of an IL at a receiver (which may result from traffic data transmissions, for example), an effective geometry factor for the TL at the TL receiver may be increased/improved, thereby improving receiver performance and/or allowing increased data rates. An effective power of an IL may be reduced using linear suppression or interference cancellation (IC) techniques, for example. Interference cancellation techniques include pre-decoder interference cancellation (pre-decoder IC), or post-decoder interference cancellation (post-decoder IC).
With linear suppression, the TL receiver includes multiple receiver (RX) antennas, and an antenna lobe is steered, by applying carefully determined delays to the multiple antenna paths, so as to point a spatial null in the direction from which a dominant source of interference is arriving. Statistics of the received signal may be used to determine the delays/combining weights leading to the desired spatial pattern, e.g., using Interference Rejection Combining (IRC) to provide improved Signal-to-Interference-and-Noise Ratio (SINR).
To significantly suppress the IL with a two-antenna receiver, the IL should arrive from a well-defined single direction. Null steering may not be effective in dispersive environments where several reflections from different directions may contribute, however. Moreover, if the null-steering degree of freedom is used to suppress the IL, this degree of freedom may no longer be available for spatial inter-symbol interference (ISI) suppression or inter-stream interference suppression in multiple-input, multiple-output (MIMO) transmissions on the TL, thus significantly lowering the equalization efficiency.
With pre-decoder interference cancellation, the receiver demodulates the IL from a received signal and applies hard decisions to the symbol estimates resulting from the demodulation, to reconstruct the transmitted symbol sequence. The reconstructed symbol sequence for the IL is filtered with the channel response to create an estimate of the received interfering signal, which is subtracted from the received signal to create an interference-reduced residual signal. After that, the desired TL signal is demodulated and decoded from the interference-reduced residual signal. With interference cancellation, the TL signal may be demodulated and decoded with higher quality than without interference cancellation.
Pre-decoder interference cancellation may be effective when the raw symbol signal-to-interference-plus-noise ratio (SINR) of the IL at the target receiver is sufficiently high to make reliable hard decisions. If the raw symbols of the IL are not sufficiently reliable, however, applying hard decisions may lead to significant decision errors and to interference amplification instead of cancellation.
With post-decoder interference cancellation, the receiver demodulates and decodes the IL from a received signal. The resulting decoded bit sequence for the IL may then be re-encoded and the coded bits passed through a modulator to reconstruct the transmitted symbol sequence for the IL. The reconstructed sequence may then be filtered with the channel response and subtracted from the received signal. After that, the desired TL signal is demodulated and decoded from the interference-reduced residual signal. Once again, with interference cancellation the TL signal may be demodulated and decoded with higher quality than without interference cancellation.
Post-decoder interference cancellation may be effective when the Modulation and Coding Scheme (MCS) applied to the IL is sufficiently conservative (with e.g., sufficiently low code rate) for the TL receiver to be able to reliably demodulate and decode the interfering signal. If radio conditions of the IL between the IL transmitter and the intended IL receiver are better than radio conditions of the IL between the IL transmitter and the TL receiver, the TL receiver may not be able to successfully decode the IL transport block. This situation may be detected using error detection/correction (such as a Cyclic Redundancy Check or CRC), which means that any degradation due to incorrect IC feedback may be avoided, but no TL geometry improvement will be achieved in this case.
Inter-cell interference is often one of the dominant impairments that limit receiver performance and the achievable data rates in cellular networks. Interference cancellation (IC) of inter-cell interference is thus an important aspect of improving data rates in throughput in cellular networks, particularly in heterogeneous networks, where cells with different coverage patterns and different downlink output powers are deployed in an overlapping manner over a network coverage area.
To apply IC to signals originating from other cells, knowledge of certain signal format parameters for the interfering signals is required to configure the receiver. For pre-decoding IC, information about resource allocation, modulation format, any pre-coding applied, the number of MIMO layers, etc., may be necessary. This information may be obtained by any of several means, including via blind estimation, eavesdropping on other-cell control signaling, or via network assistance features. For post-decoding IC, transport format parameters for the interfering signal or signals are additionally required. These transport format parameters may typically only be obtained from receiving or eavesdropping on the related control signaling info.
As already noted, heterogeneous networks (HN) differ from traditional networks in that cells with different coverage patterns and downlink output powers are deployed simultaneously over the entire network coverage area. In some cases, small cells or low-power nodes (LPNs) are placed so that they overlap with some parts of the existing macro cell coverage area, thus facilitating the offloading of some of the macro-cell UE population and increasing the overall network capacity. Some deployment schemes exist where the geographically dispersed macro and LPN cells are centrally controlled by a single base station, in a main-remote configuration, where the LPNs are the remote radio units (RRUs). Currently, a combined-cell/shared-cell scheme utilizing this concept is being standardized by the 3rd-Generation Partnership Project (3GPP).
The downlink output powers of an LPN in the HN setup (e.g., +30 dBm) may typically be 10 dB or more below that of a typical macro cell power (e.g., +43 dBm). The downlink power differences lead to an uplink/downlink coverage imbalance for the LPNs. To more fully utilize the uplink capacity increase offered by the LPNs, the network may instruct an LPN to serve UEs for which the received downlink power for the LPN is weaker than that of the macro node. The margin is controlled via a Cell Individual Offset (CIO) parameter, and may typically be around 6-10 dB. Thus, the LPN may be required to serve UEs that receive the LPN's downlink signal at a strength that is as much as 10 dB below than a received signal from the neighboring macro cell. The area where the received downlink power from the serving LPN is lower than that from a neighboring macro cell is referred to as the range-expansion (RE) region, and is illustrated in FIG. 1D as the striped region encircling the LPN near the right-hand side of the figure.
While the CIO applied to the downlink signal power measurements increases the offloading potential, it simultaneously makes it more difficult for UEs in the RE region to receive their downlink signaling and data from the serving LPN. For this reason, IC has been proposed in 3GPP as an enabling technology for RE region operation. By removing a significant fraction of the interfering macro cell power, the own-cell effective geometry for the LPN downlink is improved, and the downlink signaling and data may be received with increased reliability and throughput.